I have added a Quit option to the Finder already. If I invoke the Quit when no other apps are running, though, the Finder will simply re-launch itself (as opposed to when this preference worked, wherein the custom app would launch instead).
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user23051Mar 23 '10 at 19:54

That's funny, as I often quit Finder (see lifehacker.com/333819/add-quit-to-the-finder-menu) to get an ultra-clean desktop during presentations. After I do that, Finder will indeed relaunch automatically when all other applications are stopped, but my Mac runs just fine without Finder...
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ArjanJan 31 '10 at 11:44

Yup, quitting the Finder works perfectly fine in Snow Leopard, just as it did in Leopard. I don't want to disable it altogether; it's just somewhat annoying when it launches at login, and very annoying when it launches after all other apps quit.
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user23051Mar 23 '10 at 19:56